Why do I recommend this?

While most people aren't secretly planning for world domination, most
people may find the occasional need to hide something from even an
Administrator on their computer.
Perhaps you enjoy playing the occassional PC game which you
don't want your computer-wiz 11-year old to play.
You know, the kid who knows more about that computer than you do!
Perhaps you want to store some personal phone and address information
on your work computer.
TrueCrypt allows you to create a filesystem hidden within a strongly
encrypted file, then mounts it as a virtual disk drive when given the
correct password.
This virtual drive can be any size required - 10MB, 4GB, 100GB or more.

I actually started using such a tool on my work computer at
my previous employer ... a company which had a habit (when down-sizing)
of just showing up at your
desk, asking for your computer, and marching you to the HR office for
your severence pay.
I understood that this prevented people from damaging company
information
on the computer, but I knew it prevented me from
deleting my personal information of no use to the company.
Hiding my personal information within a 1GB encrypted filesystem was
the next best thing to being able to
delete it on my way out.